1. Field of the Invention
The present invention generally relates to an auto gain controller (AGC) and a control method thereof, and more particularly, to a three-stage dual-loop AGC and a control method thereof for adaptively controlling the gain of a radio frequency (RF) amplifier and the gain of an intermediate frequency (IF) amplifier.
2. Description of Related Art
Along with the development of technologies, television set is no more necessary for watching television programs. Instead, television programs can be displayed on any display apparatus by using a set-top box or a video card. Besides, due to the advancement of communication and compression techniques, the conventional analog television broadcasting has been gradually replaced by digital television broadcasting. A user can receive analog television signals or digital television signals through an analog tuner or a digital tuner.
For wirelessly transmitting a television signal, the television signal is first converted into an intermediate frequency (IF) signal and then into a radio frequency (RF) signal by a transmitter. Then, the RF television signal is transmitted through an antenna of the transmitter. On the other hand, a receiver receives the RF television signal transmitted by the transmitter through an antenna thereof and then converts the RF television signal into a video signal by filtering and amplifying the RF television signal. Usually, the RF television signal received by a receiver is low in voltage level or in strength, thereby being not provided directly to a demodulator. Accordingly, the RF television signal received by the receiver has to be amplified. FIG. 1 is a functional block diagram of a conventional receiver 10. Referring to FIG. 1, the receiver 10 has an antenna (not shown), a RF filter 12, a RF amplifier 14, an IF filter 16, an IF amplifier 18, and an auto gain controller (AGC) 20. The RF filter 12 is a tracking filter, which converts an input signal SIN received through the antenna into a RF signal SRF. The RF amplifier 14 amplifies the RF signal SRF and outputs an amplified RF signal SARF. The IF amplifier 18 is a surface acoustic wave (SAW) filter, which filters the amplified RF signal SARF and outputs an IF signal SIF corresponding to the desired television channel. Thereafter, the IF amplifier 18 amplifies the IF signal SIF into an IF output signal SOUT. Generally speaking, the frequency range of the RF signal SRF is wider than the frequency range of the IF signal SIF.
In order to allow the level of the IF output signal SOUT to fall within an range acceptable to the demodulator, the AGC 20 determines the strength of the input signal SIN according to the IF output signal SOUT and outputs gain control signals AGC1 and AGC2 for respectively controlling the gain of the RF amplifier 14 and the gain of the IF amplifier 18. Since one gain control signal is used for controlling the gain of the RF amplifier 14 and the other gain control signal is used for controlling the gain of the IF amplifier 18, the gain control method described above is referred to as dual-loop auto gain control.
FIG. 2 illustrates the relationship between the RF gain GRF of the RF amplifier 14 and the strength of the input signal SIN and the relationship between the IF gain GIF of the IF amplifier 18 and the strength of the input signal SIN. Referring to FIG. 2, the relationship between the RF gain GRF of the RF amplifier 14 and the strength of the input signal SIN is represented by a RF gain curve 32, and the relationship between the IF gain GIF of the IF amplifier 18 and the strength of the input signal SIN is represented by an IF gain curve 34. Besides, three different levels of strength V1, V2, and V3 of the input signal SIN are illustrated in FIG. 2, wherein V1<V2<V3.
Generally speaking, when the AGC 20 controls the gains of the RF amplifier 14 and the IF amplifier 18, the RF gain GRF does not exceed GRFmax1, and the IF gain GIF does not exceed GIFmax1. For example, when the strength of the input signal SIN is smaller than V1, the RF gain GRF and the IF gain GIF are respectively equal to GRFmax1 and GIFmin1. Besides, when the strength of the input signal SIN is between V1 and V2, the RF gain GRF remains at GRFmax1, and the IF gain GIF is gradually reduced from GIFmax1 to GIFmin1 along with the increasing strength of the input signal SIN; when the strength of the input signal SIN is between V2 and V3, the IF gain GIF remains at GIFmin1, and the RF gain GRF is gradually reduced from GRFmax1 to GRFmin1 along with the increasing strength of the input signal SIN; and when the strength of the input signal SIN is greater than V3, the RF gain GRF and the IF gain GIF are respectively equal to GRFmin1 and GIFmin1.
However, the problem of adjacent channel interference is not considered in the dual-loop auto gain control method described above. In other words, the image quality of a desired television channel may be affected when an adjacent channel has very strong signals.